Quickly converting POST requests to multi-part???

Ok, so I'm working on a WebAPI project which has many very large (60+ fields) POST requests to submit data on various forms. Currently, these POST requests are not multi-part. However, the feature I'm working on is a way to attach files to the form submission, which means they have to be converted to multi-part and have the file data added for testing purposes.

I'm finding that there are basically zero tools out there that take a non-multipart POST request and convert it into a multi-part POST. Fiddler ain't got nothing, I couldn't find anything on the web. The front-end isn't going to be modified to support this feature until later, and I have no idea how to modify it to use multi-part (setting the enctype on the FORM tag does nothing-- I'm guessing whatever JS framework they're using ignores that and builds FORM POSTs from scratch.)

So... has anybody written a quick and dirty tool for this? Is there a Fiddler plug-in? Today I manually crafted one using some Excel voodoo, it took ages, and I won't be able to use the same one tomorrow after I re-login.

If I can quickly set the POST to multi-part, I can paste-in a pre-recording of the file I'm uploading and have Fiddler make the request for me. The problem is the first step.

Oh for Christ's sake, no I'm not changing the entire design of the project. I just wanted a quick and dirty way to test my code.

If all you need is to test your back-end code with a simple upload, data tampering and transformation on the network level, plugins, 3rd party tools and excel voodoo does seem like you are on the right path for a dirty but far a way from quick.
changing the entire project design as you call it could be quicker than that.

In the case the data being submitted by the form is AS-IS and is not modified by custom submit handler.
The quick thing to do is to override or bypass, The "custom submit handler" and let the default browser handle the submit where enctype attribute is taken into account.

In the case the data being submitted by the form is AS-IS and is not modified by custom submit handler. The quick thing to do is to override or bypass, The "custom submit handler" and let the default browser handle the submit where enctype attribute is taken into account.

It's in a hipster JS MVC framework. I already tried that approach for a half hour and gave up on it.